Just yesterday I tried to run a laptop cooling fan backwards by hooking the battery up back-to-front, and it didn't start.

Do brushless DC motors actually even run backwards?

A brushless fan as found in a computer contains the controller within the fan and that controller is only designed to spin the fan in one direction. Fundamentally though, brushless DC motors are certainly reversible.

FWIW there are zillions of RC cars with brushless motors and reverse capability.

A brushless fan as found in a computer contains the controller within the fan and that controller is only designed to spin the fan in one direction. Fundamentally though, brushless DC motors are certainly reversible.

FWIW there are zillions of RC cars with brushless motors and reverse capability.

Thanks- what what I expected. Or, at least I would have been surprised if they couldn't run both ways, and thought there might be something in the motor preventing it. I wonder why though?- the plug is made to fit one way only, and since the fan is hidden inside it's not likely to get connected back to front in use.

Terry has designed broadcast stations, recording studios, broadcast equipment, intelligent machines and special computer languages for IBM, and has worked as a broadcast journalist covering elections, fires, riots and Woodstock.
He has taught electronics

Just yesterday I tried to run a laptop cooling fan backwards by hooking the battery up back-to-front, and it didn't start.

Do brushless DC motors actually even run backwards?

Jim

Inside a computer fan is a control IC - it's actually a small computer with state machine and all. This switches 3 coils on the motor, making the magnet ring turn. For feedback there is sometimes some hall-sensors too. Usually all this circuitry is built in a single component with 3 to 5 legs soldered to a small circuit board. Reversing the polarity on the wires would be like putting in power to the Arduino in reverse - you wouldn't want to do that - right ?!There is no way of reversing the fan direction without modifying it heavily - and then the fan blades do not work in reverse - look at the profile on them, fan blades and propellers only work the "right way" round.

@Zapro, yes I know the blade only works in the correct direction- I was just experimenting with reversing the fan motor because I had one and couldn't find a normal DC "toy" motor.

But I'm curious as to why they bother with the control electronics- buried inside a laptop where it won't get fiddled with under normal circumstances, and with a plug that prevents reverse polarity in the first place, it seems a lot of trouble or expense to go to.

@Zapro, yes I know the blade only works in the correct direction- I was just experimenting with reversing the fan motor because I had one and couldn't find a normal DC "toy" motor.

But I'm curious as to why they bother with the control electronics- buried inside a laptop where it won't get fiddled with under normal circumstances, and with a plug that prevents reverse polarity in the first place, it seems a lot of trouble or expense to go to.

They use brushless motors because they is more silent, and there is no brushes to wear out.

But I'm curious as to why they bother with the control electronics- buried inside a laptop where it won't get fiddled with under normal circumstances, and with a plug that prevents reverse polarity in the first place, it seems a lot of trouble or expense to go to.

Answer to both questions comes back to the folks doing the assembly. They are poorly paid labor in some place like China, or worse, who are given minimal training and have to assemble as many of these as possible in as short a time as possible. It is cheaper to make sure the unskilled labor can do it right the first time so someone else doesn't have to rework it. And there are less customers complaining about the fan not working.

Brushless DC motor requires a control circuit in order to work. The control circuit for PC fan can be made so small it easily fits inside the motor. You can find details on how a brushless DC motor works in this excellent article from Giorgos Lazaridis.